(1) Field of the Invention
The present invention lies in the field of aircraft flight controls. It relates to an adaptive flight control system for an aircraft and to an aircraft fitted with such an adaptive flight control system.
More particularly, the present invention is for a hybrid helicopter and it relates to an adaptive flight control system for piloting the pitch of blades of propulsive propellers of said hybrid helicopter.
(2) Description of Related Art
The term “hybrid helicopter” is used to designate an aircraft that combines the vertical flight effectiveness of a conventional rotary wing aircraft with the high-speed performance made possible by using propulsive propellers.
A conventional rotary wing aircraft has a fuselage and at least one main rotor driven in rotation by a power plant of the aircraft so as do provide both lift and propulsion when the rotorcraft is of the helicopter type. Such an aircraft generally includes an anti-torque device that opposes the rotor torque due to the reaction of the main rotor of the rotorcraft to the driving torque used for setting the main rotor into rotation. The rotor torque tends to cause the fuselage of the aircraft to turn in yaw in the opposite direction to the rotation of the main rotor. The anti-torque device also enables the aircraft to be piloted about its yaw axis, which is substantially parallel to the axis of rotation of the main rotor.
An anti-torque device is often constituted by an auxiliary rotor that is generally situated at the rear of the aircraft, at the end of a tail boom of the aircraft, and it is driven in rotation by the power plant of the aircraft by means of an auxiliary mechanical power transmission system.
A hybrid helicopter has a fuselage, and a main rotor driven in rotation by a power plant of the hybrid helicopter. The hybrid helicopter is also provided with a wing made up of two half-wings and with two propulsive propellers that are placed on either side of the fuselage, e.g. on respective half-wings. The propulsive propellers are driven in rotation by the power plant of the hybrid helicopter.
The main rotor serves to provide the hybrid helicopter with all of its lift during stages of takeoff, landing, and hovering flight, and generally with part of its lift during cruising flight, with the wing then also contributing part of the lift of the hybrid helicopter. Thus, the main rotor provides a hybrid helicopter in cruising flight with part of its lift and possibly also with a small contribution to its propulsion or traction forces, and it does so with minimized drag.
The propulsion of a hybrid helicopter in cruising flight is then provided mainly by its propulsive propellers. Specifically, by modifying the pitch of the blades of the propellers of a hybrid helicopter collectively by the same amount, it is possible to control the thrust generated by the propulsive propellers, and consequently to control its speed of advance.
It should be observed that the pitch of the propellers of a propulsive propeller can be varied in collective manner only, unlike the blades of a main rotor of a rotary wing aircraft in which the pitch can be varied both in collective manner and in cyclic manner. Consequently, and for simplification purposes, the term “pitch” is used below in the description to designate the “collective pitch” of the blades of a propulsive propeller.
Furthermore, the anti-torque and yaw control functions of a hybrid helicopter are implemented by using differential thrust exerted in general by the two propulsive propellers, e.g. by the pilot making use of rudder pedals.
For this purpose, the pitches of the blades of the two propulsive propellers depart from a mean value, the pitch of the blades of one propeller increasing by a differential pitch and the pitch of the blades of the other propeller decreasing by the same differential pitch. The pitch of one propeller is thus equal to the sum of the mean pitch plus the differential pitch, while the pitch of the other propeller is equal to the difference of the mean pitch minus the differential pitch. In other words, the differential pitch is equal to half of the difference in pitch of one propeller minus the pitch of the other propeller, for example.
The flight controls of a rotary wing aircraft include a cyclic pitch control stick enabling the pilot to modify the cyclic pitch of the blades of the main rotor in order to control the aircraft in pitching and in roll. A collective pitch control lever enables the pilot to modify the collective pitch of the blades of the main rotor in order to control the aircraft in elevation. Rudder pedals enable the pilot to act on the anti-torque device in order to control the aircraft in yaw. If the anti-torque device is an auxiliary rotor, the rudder pedals serve to modify the collective pitch of the blades of the auxiliary rotor. With a hybrid helicopter, the rudder pedals serve to modify the pitch of the blades of each of the two propulsive propellers in differential manner.
Furthermore, a hybrid helicopter generally includes an additional flight control constituted by a thrust lever for the propulsive propellers. The thrust lever enables the pilot to modify the pitch of the blades of both propellers in identical manner in order to modify the speed of advance of the hybrid helicopter.
Furthermore, the flight controls are generally connected to the various blades via mechanical connections referred to as “mechanical linkages” or merely as “linkages, and possibly via servo-controls, in particular on aircraft of large size for which control forces are greater.
Each mechanical linkage applies a transmission ratio, which is also known as its “gain”, between the flight control order from the pilot on the flight control and the corresponding variation in the pitch of the blades. The gain may be different for any one flight control, but it is generally constant for each flight control on a conventional rotary wing aircraft.
Nevertheless, it may be useful for the gain to be modified for certain flight controls depending on the flight conditions of the aircraft, and this applies in particular for the yaw flight control in a hybrid helicopter.
For example, when an aircraft has a high speed of advance, it is known that moving the rudder pedals through large control amplitudes can lead to maneuvers that generate large mechanical stresses on the aircraft and/or that cause the aircraft to yaw violently or even dangerously about its yaw axis. This risk, which is well known for airplanes, is just as real for hybrid helicopters having speeds of advance during cruising flight that are considerably greater than the speeds of advance of conventional rotary wing aircraft.
In order to reduce this risk, the mechanical linkage for controlling flight in yaw and corresponding to the rudder pedals may include a mixing unit. A mixing unit is a mechanical device in which, by way of example, an order from a flight control is superposed on a value of a flight parameter, or indeed in which two orders coming from distinct flight controls are superposed.
By way of example, a mixing unit may seek to modify the amplitude of a flight control depending on the speed of advance of the aircraft, as described in Document FR 2 476 013. Such a mixing unit thus enables a controlled member of an airplane, such as an elevator or a rudder, to perform a large movement at a low speed of advance, while reducing the amplitude of that movement at a high speed of advance. The mixing unit then has an adjustment member that moves as a function of a signal delivered by the dynamic air pressure on the aircraft and limits the movement of a link in the mechanical linkage of the flight control to a greater or lesser extent. The gain of the mechanical linkage for the flight control is itself constant.
A mixing unit may also modify the gain that is applied by the mechanical linkage of a flight control depending on the speed of advance of the aircraft, as described in Document FR 1 132 452. Such a mixing unit thus makes it possible for an identical movement of the flight control to give rise to different movements of a controlled member as a function of the speed of advance of the aircraft. The mixing unit has a pin belonging to a mechanical linkage of the flight control of an aircraft, and the position of the pin is modified as a function of the speed of advance of the aircraft. A change in the position of the pin serves to modify the gain of the mechanical linkage.
In addition, Document U.S. Pat. No. 2,940,332 is known, which describes a flight control system that applies variable gain to a flight control order. Gain is varied by means of an actuator deforming a parallelogram made up of links and arranged in the mechanical linkage of the flight control. The actuator can be controlled by an external setpoint associated with conditions of flight, such as the altitude or indeed the speed of the aircraft.
Although advantageous, those solutions appear to be poorly adapted to the very particular context of a hybrid helicopter, in particular because of their limitations in terms of variable gain and of lack of accuracy on the flight control output.
Furthermore, Document FR 2 946 316 is known, which relates to a hybrid helicopter and describes a mixing unit that modifies the gain applied by the mechanical linkage of a yaw flight control in compliance with the collective thrust control order for the propulsive propellers. That mixing unit includes adjustment means comprising pulleys and a belt connected to the thrust control for the propulsive propellers. That thrust control for the propulsive propellers constitutes a reliable and robust indicator of the speed of advance of the hybrid helicopter in flight relative to the air.
Specifically, an action of a pilot on the thrust control causes the belt to move and modifies the length of a lever arm in the yaw flight control mechanical linkage, thereby varying the gain of that yaw flight control mechanical linkage. Furthermore, that Document FR 2 946 316 also describes a mixing/coupling unit that enables a correction term to be added to the yaw flight control as a function of the thrust control order for the propulsive propellers. However, those mixing units are complex in terms of operation and installation. Furthermore, that mixing unit takes account of the value of the collective thrust control issued by the pilot solely for the purpose of varying the gain of the yaw control linkage. The variable gain is therefore not applied to controlling the collective thrust of the propulsive propellers. In addition, those adjustment means comprising pulleys and a belt are limited in terms of range and variation of gain and also give rise to losses of accuracy in the control output.
Furthermore, the rudder pedals, constituting the yaw flight control of a hybrid helicopter, and the thrust lever for the propulsive propellers act jointly on controlling variation of the pitch of the blades of the two propulsive propellers.
This variation in the pitch of the blades of the propulsive propellers is generally controlled initially by means of first specific linkages connected respectively to the rudder pedals and to the thrust lever, and then by two second linkages, each second linkage being connected to one of the propulsive propellers. Grouping means then serve to group the first linkages together and then to actuate the second linkages. The pitch of the blades of the propulsive propellers is then varied identically if the flight control order comes from the thrust lever. Variation is controlled differentially if the flight control order comes from the rudder pedals.
Each second linkage generally acts mechanically to control variation in the pitch of the blades of one of the propulsive propellers, e.g. by moving in translation a tube that is coaxial about the shaft for driving the blades of the propulsive propeller in rotation.
Each second linkage may also act hydraulically to control this variation in the pitch of the blades of a propulsive propeller, as described in Document FR 2 992 696. Each second linkage controls a hydraulic distributor and a hydraulic fluid, such as oil, flows inside a jacket in order to feed an actuator that generates the pitch variation. In addition, that jacket may contain the shaft for driving the blades of the propulsive propeller in rotation, which propeller may also be movable in translation along its axis of rotation as well as being movable in rotation. Furthermore, the drive shaft, which is at least partially inside the jacket, is thus at least partially surrounded by the fluid.
Finally, the technological background of the invention includes amongst other documents U.S. Pat. No. 2,620,772, which describes a variable-ratio force-amplification device for a flight control.